Help solving a UHV problem (anodized aluminum)

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    Aluminum Uhv
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Discussion Overview

The discussion revolves around a problem encountered with a 4.5 m aluminum tube intended for ultra high vacuum (UHV) applications, which has been anodized incorrectly on the internal surface. Participants explore the implications of this anodization on outgassing rates and consider potential chemical methods for stripping the anodized layer while expressing concerns about damaging the surface.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant expresses concern that anodization enhances outgassing rates and seeks advice on stripping the coating, mentioning potential chemical treatments like NaOH and sulfuric+chromic acid.
  • Another participant suggests that anodization may passivate the surface and reduce outgassing, questioning the need to strip the coating.
  • A participant clarifies that the anodization was a mistake, as it was supposed to be external, and expresses certainty that a low-cost treatment was used.
  • There is a discussion about the timeline for testing the tube once it is shipped, with one participant indicating they plan to use turbo pumps to assess the situation.
  • One participant raises a question about the inner diameter of the tube, which is stated to be 6 cm.
  • Another participant notes that outgassing will eventually stop, emphasizing the importance of vacuum and heat in accelerating this process.
  • Concerns are raised about the service temperatures for aluminum, with a participant stating they cannot exceed 180°C.
  • There is a suggestion to ask the manufacturer to correct the anodization issue before shipping, along with a question about the facilities available for etching such a long tube.
  • A participant mentions that sodium hydroxide is commonly used to remove anodized coatings on aluminum.
  • One participant indicates they are currently turbo pumping to evaluate the situation.

Areas of Agreement / Disagreement

Participants express differing views on the implications of anodization for UHV applications, with some suggesting it may be beneficial while others are concerned about its effects on outgassing. There is no consensus on the best approach to address the anodization issue, and the discussion remains unresolved regarding the optimal method for treatment.

Contextual Notes

Participants express uncertainty about the effectiveness of various chemical treatments and the potential risks involved in stripping the anodized coating. There are also limitations regarding the assumptions made about the anodization process and the specific conditions of the tube's preparation for UHV use.

Ogon Bat
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I recently realized a quite long (4.5 m) tube for ultra high vacuum in alluminium. Unfortunately in the realization a terrible error has been made... The internal surface has been anodized! I know that such treatment horribly enhance the outgassing rate... I read over the net that in principle it is possible to strip the anodized coating using some chemicals... Someone suggest NaOH solution other acid treatment (sulfuric+chromic acid)... but I'm a little bit concerned to destroy the surface, to do worst instead to improve.

Has someone some experience in doing this stuff?
 
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Ogon Bat said:
I recently realized a quite long (4.5 m) tube for ultra high vacuum in alluminium. Unfortunately in the realization a terrible error has been made... The internal surface has been anodized! I know that such treatment horribly enhance the outgassing rate... I read over the net that in principle it is possible to strip the anodized coating using some chemicals... Someone suggest NaOH solution other acid treatment (sulfuric+chromic acid)... but I'm a little bit concerned to destroy the surface, to do worst instead to improve.

Has someone some experience in doing this stuff?
I worked in UHV a long time ago when 304 stainless steel was the standard material used for UHV vacuum chambers; so I don't have any direct experience with UHV aluminum chambers. But take a look at this: https://www.atlasuhv.com/products/vacuum-chambers/aluminum-vacuum-chambers/.

Assuming the tube was specifically prepared for a UHV chamber (was it?), the anodized coating is designed to passivate the surface and reduce outgassing [that makes a lot of sense to me]. Don't strip off the coating until you check!
 
CrysPhys said:
Assuming the tube was specifically prepared for a UHV chamber
Thank you for your answer! Unfortunately no, it was a mistake. The anodizing just had to be done externally... the company, absolutely not equipped to prepare UHV components, forgot to protect the inside part. I guess (i.e. I'm sure) that they have done a quite low cost treatment, not specific for UHV. However yes it is a good idea to try first.
 
Ogon Bat said:
However yes it is a good idea to try first.
Will you be able to try it in the next day or two and let us know how it goes? Or will it be a while to put the system together for initial tests?
 
berkeman said:
Will you be able to try it in the next day or two and let us know how it goes? Or will it be a while to put the system together for initial tests?
well the tube I hope will be shipped next week and then I will try with turbo pumps to see how things are going... For now, from the net, it seems a good idea to use a chromic acid/phosphoric acid solution...
 
You mentioned that the tube is quite long (4.5 m). What is the inner diameter?
 
All out-gassing stops eventually. A combination of vacuum and heat will accelerate that process.
 
Dullard said:
All out-gassing stops eventually. A combination of vacuum and heat will accelerate that process.
Well, that depends on how long "eventually" is. That's what distinguishes UHV. You need to be concerned about the outgassing rates and service temperatures (I baked my systems at 300 deg C) of the materials used for the chamber and used for the components within the chamber.
 
Last edited:
The inner diamater is 6 cm. For alluminium I cannot go over 180°C
 
  • #10
Ogon Bat said:
well the tube I hope will be shipped next week and then I will try with turbo pumps to see how things are going... For now, from the net, it seems a good idea to use a chromic acid/phosphoric acid solution...
I don't quite understand. If the manufacturer messed up, and hasn't shipped yet, why don't you ask them to correct the problem (assuming it is a problem)? Otherwise, assuming you do need to etch away a crappy coating, do you have the facilities to handle etching of such a large (in particular, long) tube?
 
  • #11
Sodium hydroxide is commonly used to remove anodized coatings on aluminum.
 
  • #12
well now I'm turbo pumping... let's try to see how disastrous the situation is.
 

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